The recording of seismic signals generated during seismic exploration activities has traditionally required the sesimic data from remote sensor to be transmitted over miles of interconnecting cables to a centralized recording system. This method of performing a seismic survey is severely limited in areas of rugged terrain: mountainous regions, jungles, and anywhere surface obstructions make the cable interconnections difficult and impractical. Furthermore, in most seismic recording systems it is desirable to utilize multichannel recording equipment in which each channel consists of a multigeophone array. A multigeophone array is a grouping of individual geophones connected along a cable. Most seismic data are recorded from groups of multigeophone arrays, rather than single geophones or arrays. There are usually twenty-four, forty-eight, or ninety-six groups on a cable. Each group puts one sequence of electrical signals into the cable. The signals go through individual wires in the cable to the recording/processing instruments. Spreads composed of multigeophone arrays increase the complexity and cost of the cables and the frequency of their repair and maintenance.
Advances in instrumentation have made it possible to distribute portions of the data processing and data recording functions to wireless ("isolated") recording stations ("isolatted distributed recorders"). These "Isolated Distributed Recording Systems" eliminate the need for complex cable interconnections and provide greater flexibility in field operations.
In place of the communications link provided by an interconnecting cable system, Isolated Distributed Recording System generally utilize a Radio-Telemetry System or a Time Synchronization System. These systems may be briefly described as follows:
Radio-Telemetry Systems utilize a coded radio command to activate the isolated distributed recorders. To synchrozine the seismic data recording function and the seismic signal transmitting function, one with the other, timing pulses may also be radio-transmitted to the isolated distributed recording units. Once recorded, the seismic data may be played via radio to a central recorder station for permanent storage. Alternatively, the data may be recorded at the isolated distributed recorder. The data may be stored within the lithographed microcircuitry of a silicon chip, or a magnetic disc or tape may be used as the storage medium at the isolated distributed recorder. Once recorded, the seismic data can be retrieved from each isolated distributed recorder and played back at the central recorder station.
Representative Radio-Telemetry Systems are disclosed in Herzog U.S. Pat. No: 3,062,315; Aitken U.S. Pat. No. 3,075,607 et. al.; Montogomery U.S. Pat. No. 3,283,295; Lobe U.S. Pat. No. 3,288,242; Broding U.S. Pat. Nos. 3,506,864, 3,987,406, and 4,010,442; Kostelnicek U.S. Pat. No. 3,886,494; Winstein U.S. Pat. No. 3,946,357; and Ezell U.S. Pat. No. 4,042,906.
Time Synchronizations Systems, on the other hand, utilize an accurate time reference and a pre-selected time recording logic to operate the isolated distributed recorders independently of the central control station. Representative Time Synchronization System are disclosed in Siems U.S. Pat. Nos. 4,281,403,et al; Bassett U.S. Pat. No. 3,972,019; and Pelton U.S. Pat. No. 3,733,584, et al.
The pre-selected time recording logic eliminates radio transmissions of data between each isolated unit and the central control station. Recording of seismic data at each isolated distributed recorder is either continuous, usually for one day while a seismic survey is being "shot", or the recording of seismic data is effected in accordance with a pre-selected time logic at selected time periods regardless of whether seismic signals are being generated.
Although Radio-Telemetry Systems and Time Synchronization Systems represent a substantial contribution to the art of geophysical prospecting, these systems do suffer from certain disadvantages.
Radio-Telemetry Systems, for example, suffer from difficulties in reliably receiving the radio frequencies that are usually allocated to this type of service. To a large extent this has resulted from the difficulties inherent in separating the individual radio waves emanating from each separating the individual radio waves emanating from each geophone station and separately amplifying them without serious "cross-talk" or distortion. Cross-talk occurs when a portion of one signal mixes with or overlays another signal. This difficulty is accentuated by the fact that each individual distributed recorder or groups of recorders in a seismic plan are usually addressed by a different encoded radio command thereby increasing the difficulties and complications associated with the use of Radio-Telemetry Systems. Also, radio-controlled systems may use radio frequencies that are effective only along line-of-sight, and, therefore, such systems are often not effective in mountainous or obstructed terrains. Finally, in some areas, it is difficult, if not impossible, to obtain permits for use of radio transmissions, and Radio-Telemetry Systems cannot be employed.
A disadvantage of Time Synchronization Systems lies in the separation of the recording function and the seismic signal transmitting function. Said functions are independent of each other. In a Time Synchronization System the recording periods are preselected. Transmission of the seismic signal must coincide in time with the preselected recording period for any successful data gathering. Storage capacity is usually wasted during inactive shooting periods. Therefore, larger than required storage capacity at each isolated unit is usually required. Furthermore, flexibility of field operations is dependent on a pre-selected time recording logic which means that a signal must be transmitted while the recorder is in operation or such signal will not be recorded.
There exists a need, therefore, for a method and an apparatus to initiate recording at isolated distributed recorders in an Isolated Distributed Recording System which does not require radio transmissions or complete dependency on a pre-selected recording time logic to control the recording of seismic data during seismic exploration activities. It is also desired that recording of seismic data in an isolated distributed recorder occur only during periods of active "shooting" and not at any other time.